As display technology evolves, the pixel circuit plays an increasingly important role in managing the instability of the compensation process of the active matrix organic light-emitting diode (AMOLED) display technology. One factor in the instability of the compensation process is the influence of a threshold voltage (Vth) in a compensation device, or of a circuit voltage drop (IR drop). A relatively long channel is needed in designing a driving thin film transistor (TFT) to improve the electrical stability of devices. An S-type or V-type structure with a channel length of more than 40 μm is commonly used to improve the component stability. However, increases to channel length cause the amount of required layout area to increase accordingly.
As the display technology has improved, the number of pixels per unit has increased. Accordingly, minimum device size required to include a given number of pixels is reduced. The above-mentioned channel scheme cannot achieve the reduced minimum device size. When the channel length is reduced to less than 2 μm, a serious short-channel effect occurs.
It is known in the art that to reduce the layout area of a channel, a vertical channel is introduced within a limited layout area to extend the channel length. The short-channel effect can be avoided accordingly. It is disclosed in the art that, by leveraging the characteristics of a bottom-gate TFT device, a profile of a gate electrode can be used as a carrier layer of a channel. Consequently, a relatively long channel with a vertical channel structure is obtained.
In the process of fabricating a low temperature polysilicon (LTPS) TFT, the polysilicon film layer is formed as required from amorphous silicon through laser annealing. As compared with the conventional amorphous silicon TFT, the LTPS TFT approach provides an increased electron mobility, a reduced material cost, an enhanced aperture ratio, a lowered panel weight, and an improved power consumption.
However, the quality of the laser annealing may be reduced due to the bottom-gate TFT structure. As a result, the electron mobility may be affected. Although the bottom-gate TFT structure can accommodate the vertical channel scheme to increase the channel length per unit area, the channel conductivity is adversely affected.
Therefore, it is desirable to eliminate the shortcomings in the above-mentioned conventional approaches. As such, the present invention provides a vertical thin film transistor with a top-gate structure to meet the requirements of both the vertical channel design and the laser annealing process.